By late summer, Lake Hartwell showed the cumulative effect of an extreme drought. The drought conditions had a significant impact on temperatures during August.

Author's Note: The following report has not been subjected to the scientific peer review process.

Introduction

The month of August, 2007, was the warmest August on record across the western Carolinas. All three first-order stations (Asheville, Charlotte, and the Greenville-Spartanburg metropolitan area) broke the previous records for highest average temperature for the month of August. In fact, it was the warmest month ever recorded at Greenville-Spartanburg, where daily temperature records date back to the Autumn of 1917. It was the third warmest month ever at Charlotte, where daily temperature records date back to 1878.

Here is how August 2007 stacked up against previous years in terms of record monthly average temperature.

Location

New Record

Old Record

Greenville - Spartanburg Airport

84.7

81.6 (1954)

Charlotte - Douglas International

83.7

81.9 (1900)

Asheville Regional Airport

76.7

76.5 (1983)

The new record at Greenville - Spartanburg was remarkable in that it was the warmest month ever, surpassing the old record of 83.2 degrees set back in July of 1993.

Numerous daily high temperature records were tied or broken at each location.

Greenville-Spartanburg International Airport

Date

New Record

Old Record

7

100

99 (1935)

8

104

98 (1951)

9

104

98 (1980)

10

105

98 (1956)

11

100

99 (1999)

15

100

100 (1995)

16

102

100 (1954)

17

100

100 (1954)

21

102

102 (1983)

The 105 degree reading on Friday, 10 August, broke the all-time high temperature record for the Greenville-Spartanburg metropolitan area. The old record was 104 degrees, which had been set on three separate occasions: 29 July 1952, 27 June 1954, and 31 July 1999. In addition, the period from the 7th through the 11th tied the longest streak of consecutive days with a high temperature 100 degrees and above, last set on 28 June 1952. The high temperature also reached 102 degrees on 22 August. Although this was not a record, it was the tenth day in the month where the temperature reached at least 100 degrees, which shattered the previous monthly record of five days set back in June and July of 1952. The ten days so far this year tied the previous record of ten 100 degree plus days set back in the Summer of 1952. Since records began back in 1917, there have been only 86 days where the temperature reached at least 100 degrees including this year, which means that over 10% of the 100 degree days occurred this past August.

The 2007 summer season, defined as the months of June, July, and August, was the third warmest on record at Greenville - Spartanburg, with an average temperature of 79.9 degrees. This was remarkable considering that July was below normal. The mean daily temperature showed a dramatic increase during the first week of August (Fig. 1). The seven day period beginning 4 August was a particularly brutal stretch, including the five consecutive days of high temperatures at or above 100 degrees (Fig. 2).

Figure 1. Daily mean temperature at the Greenville-Spartanburg Airport for the period 9 June to 7 September 2007. Click on image to enlarge.

Figure 2. Daily high temperature at the Greenville-Spartanburg Airport for the period 9 June to 7 September 2007. Click on image to enlarge.

Charlotte - Douglas International Airport

Date

New Record

Old Record

8

102

101 (1951)

9

104

100 (1951)

10

104

100 (1956)

22

101

101 (1983)

The 104 degree temperatures on the 9th and 10th broke the previous record high temperature for August, which was 103 degrees, and tied the all-time high temperature last set on 6 September 1954. The temperature surpassed 100 degrees on the 16th and 21st as well. The six days of high temperatures at least 100 degrees broke the old record for the month of four, set back in 1954 and 1983. The high temperature was above 90 degrees every day at the Charlotte - Douglas International Airport (Fig. 3). In addition, it was the driest August on record. Only 0.41 inches of rain fell, which was less than the old record of 0.61 inches back in 1972 (Fig. 4).

Figure 3. Daily maximum temperature at the Charlotte - Douglas International Airport for the period 9 June to 7 September 2007. Click on image to enlarge.

Figure 4. Daily precipitation at the Charlotte - Douglas International Airport for the period 9 June to 7 September 2007. Click on image to enlarge.

Asheville Regional Airport

Date

New Record

Old Record

9

94

93 (1983)

15

93

93 (1995)

16

94

93 (1995)

The temperature reached 90 degrees or greater for 16 consecutive days at the Asheville Regional Airport, which was the longest streak since 1993.

The making of a heat wave

Several factors contributed to the hot temperatures in August 2007, not the least of which was a long term (and ongoing) extreme to exceptional drought across much of the Southeast (Fig. 5), which began in earnest across the western Carolinas in January. Through the end of August, large portions of the Southeast had yearly rainfall deficits on the order of 12 to 18 inches (Fig. 6), which was only about 50% to 70% of normal (Fig. 7). In fact, the Southeast Region had the driest 8 month period on record (Fig. 8).

Figure 5. U.S. Drought Severity as of 28 August 2007. Click on
image to enlarge.

Figure 6. Accumulated precipitation departure from normal for the
period 1 January to 31 August 2007. Click on image to enlarge.

Figure 7. Percent of average precipitation from 1 January through the end of August 2007. Click on image to enlarge.

Figure 8. Precipitation for the January-August period, 1895 to 2007. Click on image to enlarge.

A long term drought has severe consequences for soil moisture. By the end of August, most of the area from the Piedmont of the Carolinas to the southern Appalachians ranked near the first percentile in terms of soil moisture ranking (Fig. 9). This can be interpreted as soil conditions being almost as dry as they have ever been.

Figure 9. Soil moisture percentile ranking as of 31 August 2007. Note the large area with a ranking of '1' or less across the Southeast.

A portion of the energy from the sun is used to evaporate moisture from the top of the soil. This energy is converted into latent heat, which means it is stored in the water vapor and will be released later when the vapor condenses back into water droplets. As soil moisture drops, there is less moisture available to "use up" energy through evaporation. Furthermore, there is less water available for plants to use which leads to less transpiration. The result is that more of the sun's energy is used to heat up the air, so temperatures tend to be warmer. In other words, given the same atmospheric conditions and sunlight, the air will be hotter over a very dry soil and relatively cooler over a wet soil.

By late July, soil moisture conditions were perfect for a heat wave as soon as the weather pattern supported excessive heat. This finally happened in the first few days of August, when a large upper ridge of high pressure centered over the southern Plains drifted eastward over the Southeast. By the morning of 7 August, an upper anticyclone at 500 mb was centered over Georgia. For the next few days, the upper air pattern featured a persistent anticyclone over the Southeast, until it finally moved back west to the southern Plains on 11 August. The upper ridge acted to suppress the development of showers and thunderstorms during this period. At 850 mb, a high pressure ridge moved over the Deep South on 5 August and persisted through 10 August. The west to northwest wind at this level resulted in a downslope flow east of the mountains, which warmed low levels by adiabatic compression. Furthermore, the prevailing wind acted to cut off the western Carolinas from any moisture sources.

Given the lack of soil moisture, there was not enough moisture in the lower atmosphere to support anything more than isolated showers through 10 August. An upper air sounding taken at Greensboro, North Carolina, in the evening of 9 August showed this best (Fig. 10). Although surface temperatures were in the upper 90s at the time, the atmosphere was only weakly unstable because of the lack of moisture.

Figure 10. Skew T - log P diagram for the upper air sounding taken at Greensboro, North Carolina, at 8 pm on 9 August. The red line is the temperature sounding and the green line is the dew point sounding. Winds aloft are shown as brown barbs on the right side of the figure. A table of convective parameters is shown on the right. Click on image to enlarge.

Discussion and Summary

Given the long term and worsening drought during the first part of the Summer, it should come as no surprise that temperatures would be above normal as the season progressed, given a weather pattern that encouraged sunshine and discouraged the development of showers and thunderstorms. Extreme drought conditions favor excessive heat. The astute reader might also make a connection between some of the other dry years in Figure 8, and some of the previous high temperature records across the Piedmont, especially those set in 1954.

In an extreme drought, there is less water available for evaporation, which does not allow the lowest part of the atmosphere to become as humid as it typically would be if soil moisture was normal. The result is a more stable atmosphere, which can lead to a decrease in shower and thunderstorm activity in the absence of any cold front passages. This in turn means less rainfall. In that respect, it can be argued that drought begets more dry weather. Until a significant shift happens in an overall weather pattern, or the arrival of some larger, synoptically driven low pressure system such as the remnants of a tropical cyclone, a drought will tend to perpetuate itself.

As of early September, the prospects for a quick end to the drought do not appear encouraging. It is estimated that somewhere between 12 to 18 inches of rain is needed to end the drought by the onset of Winter (Fig. 11). The most recent seasonal drought outlook calls for only limited improvement across the western Carolinas and northeast Georgia (Fig. 12).

Figure 11. Amount of precipitation needed to bring an end to the drought, issued 8 September 2007. Click on image to enlarge.

Figure 12. U.S. seasonal drought outlook for September through
November, 2007.

Acknowledgements

John Tomko contributed to this review. Figures 1-4 were created from the CRONOS database from the State Climate Office of North Carolina. The upper air sounding was obtained from the significant events database at the Storm Prediction Center.